1. Field of the Invention
The present invention relates to an optical recording medium including a substrate and a light transmitting layer having a smaller thickness than that of the substrate.
2. Description of the Related Art
A substrate of an optical recording medium is generally formed by injection molding so as to transfer an information pattern such as pits and grooves thereon. Therefore, it is important that a material of the substrate is excellent in moldability and transferability. Moreover, for the following reason, a material excellent in moldability and transferability as well as in light transmittance is generally used as a material of the substrate for an optical recording medium.
For example, a CD (Compact Disc) includes a substrate having a thickness of 1.2 mm that also serves as a light transmitting layer. Therefore, polycarbonate or the like, which is excellent in light transmittance, is used as a material of the substrate.
On the other hand, a DVD (Digital Versatile Disc) includes, for example, a substrate having a thickness of 0.6 mm and a dummy substrate (a substrate serving to maintain the rigidity) having a thickness of 0.6 mm, which are bonded to each other, as an example. If a material of the substrate differs from that of the dummy substrate, a warp is likely to occur due to a sudden change in ambient temperature within a short period of time. Therefore, the same material is generally used for the substrate and the dummy substrate. Since polycarbonate or the like having excellent light transmittance is used as a material of the substrate, polycarbonate or the like is also generally used as a material of the dummy substrate.
In recent years, in order to remarkably increase a recording capacity, blue-violet laser light having a short wavelength of about 405 nm is used while a numerical aperture of an objective lens is increased to about 0.85. In order to follow such improvement, an optical recording medium including: a substrate having a thickness of about 1.1 mm; and a light transmitting layer formed on the substrate so as to have a thickness of about 0.1 mm, which is smaller than that of the substrate, has attracted attention (for example, see Japanese Patent Laid-Open Publication No. 2003-85836). The optical recording medium has a gap of about 150 nm with a head, and therefore has a remarkably reduced gap as compared with a CD or a DVD having a gap of about several mm with a head.
The optical recording medium is fabricated by injection molding a substrate, and forming a light transmitting layer by spin-coating or bonding a light transmitting film on the substrate. Since a fabrication method of the substrate or the like differs from that of the light transmitting layer, the substrate and the light transmitting layer differ from each other not only in thickness but also in material in many cases.
As a material of a substrate, polycarbonate or the like is used, because polycarbonate meets performance targets as a material of the substrate for a conventional optical recording medium and has reliability in view of moldability and transferability. On the other hand, as a material of the light transmitting layer, a light transmitting film made of a radiation curable resin cured by a radiation ray such as an ultraviolet ray or an electron beam is used.
However, since the optical recording medium has an asymmetric structure composed of the substrate and the light transmitting layer which differ from each other in material as well as in thickness, there arises a problem that a warp is likely to occur due to a sudden change in ambient temperature within a short period of time and the like. If the warp is large, the optical recording medium is brought into contact with a head or a gap with the head becomes too large or too small to easily cause an error in information recording/reading. For this reason, it is necessary to restrain the warp within a predetermined limit value.
On the other hand, if a metal or glass is used as a material of the substrate, the warp can be kept within a predetermined limit value. However, such a material is not preferred in view of productivity and cost.
If a thickness of the substrate is increased or a rib or the like is formed on the substrate, the rigidity of the substrate can be increased thereby so as to restrain the warp. At the same time, however, such a change in shape of the substrate also requires a change in design of a recording/reading device, and therefore, is not preferred in view of compatibility and cost.
It is also conceivable to increase the rigidity by adding a predetermined filler in a material of the substrate. However, the addition of a filler degrades the moldability of a substrate and the transferability of an information pattern, resulting in the opposite effect to adversely affect electrical characteristics such as a degraded jitter and an increased number of errors.